Interferon-induced enzyme activities such as the oligo(2' yields 5')adenylate synthetase, the 67K dalton protein kinase and oligo(2' yields 5') A phosphodiesterase are investigated with a goal of understanding their role in the action of interferon, the induction of interferon by double-stranded RNA and, perhaps, control of cell growth and differentiation. Analogs of the mediator of interferon action, 2-5A, are synthesized in order to define the relationship between oligonucleotide structure and binding to and activation of the 2-5A dependent endonuclease with the eventual goal of designing useful chemotherapeutic agents based on this system. Considerable progress has been made in gaining an appreciation of the factors that influence binding of each of the individual bases of the 2-5A molecule to the 2-5A-dependent endonuclease. The N6 amino group of the first (or 5'-terminal) adenosine residue of 2-5A trimer is critical in RNase L binding whereas the N6 amino moiety of the third (or 2'-terminal adenosine residue of 2-5A is crucial for RNase L activation. The second adenosine unit of 2-5A does not seem to be critical for either binding or activation. In regard to the purine N7 moiety of the basis of 2-5A, activation of mouse L, but not binding to mouse RNase L depended on the presence of the first and third adenosine residues of 2-5A, but the N7 moiety of the second adenosine was not essential for either binding or activation. Sequence-specific 8-bromo analogs of 2-5A provided one analog with ten times the biological activity of natural 2-5A, and another analog which did not require the presence of a 5'-di- or triphosphate moiety to activate RNase L.